AUTODETACHMENT LIFETIMES, ATTACEIMENT CROSSSECTIONS, AND NEGATIVE IONS psec, and at higher concentrations the decay kinetics become first order in sulfite. The rate constant for (17) eaq-
+ S032-+product
(17)
is estimated from the half-life of 9 Nsec in 50 mM S032- to k17 I 1.5 X loe M-l sec-l, which is in M-' sec-I found by agreement with 1.3 X Anbar and Hart.a The reason for examining eaqin Ar-saturated solution was the possible exclusion of the reaction 0503'- +sod2-f eaq (18)
+
which would replace reaction 1. There are no indications at all at 700 nm of an additional ea4- production in Ar-saturated solution even with 40 mM SO?-, so we are justified in leaving the reaction out of consideration. Furthermore, reaction 18 would be in contradiction to the proposed chain mechanism in 0 2 solutions. We feel confident that the absorption at 700 nm in the Ar-saturated solution is due to eaq- absorption (measured E = 1.5 X lo4 M-' cm-l) and not to the species HS032- (or Soa3-)as proposed by Adams and Boag6 as responsible for the absorption at 720 nm in oxygen-saturated solutions. Dogliotti and Hayon6 have not observed the transient in N%O-saturated solutions, but, on adding N20, they have found an in-
3517
crease of more than a factor of 2 over the transient at 275 nm. The reason for the observation in airsaturated solutions is that the thermal oxidation of sulfite with oxygen in neutral media is so fast that the oxygen is consumed immediately. Recalculation of = 1.58 X the data in Figure 1 (ref 6) with 104 M-l cm-I 26 gives an E of about 1000 M-' cm-l for the absorption at 275 nm compared with ours of 1300 M-l cm-l at 260 nm. The first-order rate constant reported k = 1.9 X lo4 sec-l in 20 mhf !%'for the species absorbing at 720 nm, may be recalculated as a pseudo first order giving k(e,,S032-) = 1 X lo6 M-l sec-l compared with that of Anbar and Harts of 1.3 X IOe M-' 8ec-l and ours of 1.5 X lo6 M-l sec-'. From a competition study in which they used NO3- ions, Dogliotti and Hayong have estimated SOB'-) I , 2 X lo6 M-' the rate constant .k(eaqsec-'. Acknowledgments. We are indebted to E. J. Hart and H. Fricke for discussions of this work. We wish to thank 0. I,. Rasmussen for performing the computer calculations. We greatly appreciate the skillful assistance of the accelerator staff, and one of us (Z. P. 2 . ) is indebted to the Danish Atomic Energy Commission for the fellowship granted.
+
+
(26) E. J. Hart, Science, 146, 19 (1964)
Autodetachment Lifetimes, Attachment Cross Sections, and Negative Ions Formed by Sulfur Hexafluoride and Sulfur Tetrafluoride by P.W. Harland and J. C. J. Thynne*l Chemistry Department, Edinburgh University, Edinburgh, Scotland
(Receiued December $1, 1PYO)
Publication costs borne completely by The Journal of Physical Chsmistry
The formation of long-lived temporary negative ion states in sulfur tetrafluoride and sulfur hexafluoride has been studied. The autodetachment lifetimes of SF,-* and SFe-* are 16.3 & 0.3 and 68 .rt 2 psec, respectively. The measured ratio of the cross sections for electron attachment to SFs and SFI was 109 =t6; this leads to a value of 10.7 0.6 X 10-17 cm2 for the electron attachment cross section of sulfur tetrafluoride. This is some two orders of magnitude smaller than the cross section of the hexafluoride. The SF,-F bond dissociation energy has been estimated to be G3.6 eV and a value of 2.9 i 0.1 eV deduced for the electron affinity of sulfur trifluoride.
*
Introduction When an electron interacts with a molecule, one of several processes may occur depending on the energy of the electron and the nature of the molecule. Of these, the processes which lead to negative ion formation are conventionally classified as (i) resonance capture,
which occurs with low-energy electrons (usually 0-2 ev) AB e --+ AB-
+
(1) Correspondence should be addressed t o Department of Trade and Industry, Room 506, Abell House, John Islip Street, London S. W. 1, England. The Journal of Physical Chemistry, Vo1. 76,N o . $3, 1971
P. W. HARLAND AND J. C . J. TIIYNNE
3518
(ii) dissociative capture, the relevant electron energy range being -0-10 eV
AB
+ e +A- + B
(iii) ion-pair formation which usually occurs with electrons of energies > 10 eV
AB
+e
--f
A-
+ B+ + e
As part of a study of negative ion formation by inorganic fluorides2r3 we have examined sulfur tetrafluoride and sulfur hexafluoride. The latter molecule has been studied by several s ~ o r k e r swho ~ ’ ~have noted an abundant parent ion SFs- to be formed at 0 eV, and this ion is frequently used to calibrate the electron energy scale. Sulfur tetrafluoride has not been previously examined for negative ion formation. In electron impact studies, when the electrons are emitted from a heated filament, because of the energy spread of the ionizing electrons uncertainties may arise in the determination of the experimental ionization efficiency curves, the appearance potentials becoming l( smeared out.” To reduce the effects of this electron energy distribution analytical methods have been developed for positive6 and negative ion7studies. It has been shown recently that sulfur hexafluoride attaches an electron with a large cross section to form a relatively long-lived metastable ion whose lifetime may be investigated by a time-of-flight t e ~ h n i q u e . ~I ,n~ preliminary experiments we also observed the formation of a temporary negative ion state with sulfur tetrafluoride] and accordingly we have compared the attachment cross sections and metastable lifetimes of both fluorides. Experimental Section The experiments were performed on a Bendix timeof-flight mass spectrometer] Model 3016. Ion source pressures were usually maintained below 5 X Torr (
